Waveguide including volume bragg gratings

ABSTRACT

A waveguide is provided for conveying image light. The waveguide includes an input port for receiving a first beam of image light carrying an image in a wavelength band. A first diffraction grating of the waveguide includes a plurality of volume Bragg gratings (VBGs) configured to expand the first beam along a first axis and to redirect the first beam towards a second diffraction grating of the waveguide. The second diffraction grating includes a plurality of VBGs configured to receive the first beam from the first diffraction grating and to out-couple different portions of the first wavelength band of the first beam along a second axis, thereby expanding the first beam along the second axis for observation of the image by a user.

REFERENCE TO RELATED APPLICATIONS Technical Field

The present disclosure relates to optical components and modules, and inparticular to optical waveguide based components and modules usable indisplay systems.

Background

Head-mounted displays (HMDs), near-eye displays, and other kinds ofwearable display systems can be used to provide virtual scenery to auser, or to augment a real scenery with additional information orvirtual objects. The virtual or augmented scenery can bethree-dimensional (3D) to enhance the experience and to match virtualobjects to the real 3D scenery observed by the user. In some displaysystems, a head and/or eye position and orientation of the user aretracked in real time, and the displayed scenery is dynamically adjusteddepending on the user's head orientation and gaze direction, to provideexperience of immersion into a simulated or augmented 3D environment.

Lightweight and compact near-eye displays reduce the strain on user'shead and neck, and are generally more comfortable to wear. The opticscan be the heaviest module of the display. Compact planar opticalcomponents, such as waveguides, gratings, Fresnel lenses, etc., can beused to reduce size and weight of an optics block. However, compactplanar optics may have limitations related to image resolution, imagequality, ability to see the real world through the display, field ofview of generated imagery, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described in conjunction with thedrawings, in which:

FIG. 1A is a plan (XY plane) view of a near-eye display (NED) includinga waveguide of this disclosure;

FIG. 1B is a side cross-sectional view of the NED of FIG. 1A;

FIG. 2A is a plan ray-trace view of an embodiment of the waveguide ofFIGS. 1A and 1B including reflective volume Bragg gratings (VBGs) in thetop grating;

FIGS. 2B and 2C are side cross-sectional cutout views of the waveguideof FIG. 2A along lines B-B and C-C respectively;

FIG. 3 is an example diffraction efficiency spectrum of a single volumeBragg grating (VBG) usable in waveguides of this disclosure;

FIG. 4A is a side cross-sectional view of a waveguide including a VBGlayer;

FIG. 4B is a plot of FOV vs. wavelength for different VBG periods;

FIG. 5 is a side cross-sectional view of an NED including a waveguide ofthis disclosure, illustrating a principle of pupil expansion bywavelength division;

FIG. 6 is a plan ray-trace view of an embodiment of the waveguide ofFIGS. 1A and 1B including transmissive diffraction VBGs in the topgrating;

FIGS. 7A and 7B are maximum and minimum VBG period maps of diffractiongratings of the waveguide of FIG. 6;

FIG. 8 is a plan ray-trace view of a waveguide configured to carry threecolor channels injected at different locations on the waveguide, the topgratings including reflective diffraction VBGs;

FIG. 9 is a plan ray-trace view of a waveguide configured to carry threecolor channels injected at different locations on a waveguide, the topgratings including transmissive diffraction VBGs;

FIG. 10 is a plot of waveguide size vs. diagonal field of view (FOV);

FIG. 11A is a plan view of a near-eye display (NED) including awaveguide having two input ports for two FOV halves;

FIG. 11B is a side cross-sectional view of the NED of FIG. 11A;

FIG. 12 is a plan ray-trace view of an embodiment of the waveguide ofFIGS. 11A and 11B, the top gratings including reflective diffractionVBGs sending light from left projector to right FOV half, and viceversa;

FIG. 13A is a plan ray-trace view of a waveguide configured to carrythree color channels injected at different locations on a waveguide, thetop gratings including reflective diffraction VBGs configured forsending light from left projector to right FOV half, and vice versa;

FIG. 13B is a plan ray-trace view of a waveguide configured to carrythree color channels injected at different locations on a waveguide, thetop gratings including transmissive diffraction VBGs configured forsending light from left projector to right FOV half, and vice versa;

FIG. 13C is a plan ray-trace view of another embodiment of a waveguideconfigured to carry three color channels injected at different locationson the waveguide, the top gratings including transmissive diffractionVBGs configured for sending light from left projector to right FOV half,and vice versa;

FIG. 13D a plan ray-trace view of an embodiment of a waveguideconfigured to carry three color channels injected at different locationson the waveguide, the top gratings including reflective diffraction VBGssending light from left projectors to left FOV half, and light fromright projectors to right FOV half;

FIG. 13E a plan ray-trace view of another embodiment of the waveguide ofFIG. 13D;

FIG. 13F a plan ray-trace view of an embodiment of a waveguideconfigured to carry three color channels injected at different locationson the waveguide, the top gratings including transmissive diffractionVBGs sending light from left projectors to left FOV half, and light fromright projectors to right FOV half;

FIG. 13G a plan ray-trace view of another embodiment of the waveguide ofFIG. 13F;

FIGS. 14A to 14C are side cross-sectional schematic views showingdifferent types of rainbow artifacts;

FIG. 15A is an isometric view of an eyeglasses form factor near-eyeAR/VR display incorporating an optical waveguide of the presentdisclosure;

FIG. 15B is a side cross-sectional view of the display of FIG. 15A; and

FIG. 16 is an isometric view of a head-mounted display (HMD)incorporating an optical waveguide of the present disclosure.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives and equivalents, as will be appreciatedby those of skill in the art. All statements herein reciting principles,aspects, and embodiments of this disclosure, as well as specificexamples thereof, are intended to encompass both structural andfunctional equivalents thereof. Additionally, it is intended that suchequivalents include both currently known equivalents as well asequivalents developed in the future, i.e., any elements developed thatperform the same function, regardless of structure.

As used herein, the terms “first”, “second”, and so forth are notintended to imply sequential ordering, but rather are intended todistinguish one element from another, unless explicitly stated.Similarly, sequential ordering of method steps does not imply asequential order of their execution, unless explicitly stated. In FIGS.1A, 1B, 2A-2C, 5, 6, 8, 9, 11A, 11B, 12, and 13A-13G, similar elementsare denoted with similar reference numerals.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments andmodifications, in addition to those described herein, will be apparentto those of ordinary skill in the art from the foregoing description andaccompanying drawings. Thus, such other embodiments and modificationsare intended to fall within the scope of the present disclosure.Further, although the present disclosure has been described herein inthe context of a particular implementation in a particular environmentfor a particular purpose, those of ordinary skill in the art willrecognize that its usefulness is not limited thereto and that thepresent disclosure may be beneficially implemented in any number ofenvironments for any number of purposes. Accordingly, the claims setforth below should be construed in view of the full breadth and spiritof the present disclosure as described herein.

In accordance with an aspect of this disclosure, a waveguide may includea diffraction grating configured for pupil expansion along one axiscoupled to another diffraction grating configured for pupil expansionalong another, e.g. perpendicular axis, and for out-coupling of he lightfor observation by a user. The user may observe the external worldthrough the second grating. To reduce haze caused by multiple volumeBragg gratings (VBGs) of the second grating, the latter may beconfigured to output different wavelengths of a same color channel ofthe image to be displayed at different locations along the seconddirection, thereby reducing required VBG density and associated haze. Inother words, the pupil is expanded in the second direction by wavelengthdivision.

In accordance with an aspect of the present disclosure, a waveguide forcarrying image light may include multiple input ports for receivingoptical beams each carrying a portion of field of view (FOV) of theimage being displayed. Grating structures of the waveguide may beconfigured to expand output pupil of the waveguide while out-couplingthe portions of the FOV for observation by a user in such a manner thatthe user perceives a single, large FOV. The FOV can be increased byusing multiple input ports.

In accordance with the present disclosure, there is provided a waveguidefor conveying image light carrying an image having a field of view(FOV). The waveguide includes first and second input ports for receivingfirst and second beams of image light carrying first and secondportions, respectively, of the FOV of the image; opposed first andsecond outer optical surfaces for propagating the first and second beamstherebetween; and a first diffraction grating configured to expand thefirst and second beams along a first axis, wherein the first and secondbeams are out-coupled from the waveguide for observation of the firstand second portions of the FOV of the image by a user. The first andsecond portions of the FOV may be conterminous or partially overlapping.The first and second input ports may be coupled to the first opticalsurface on opposite sides of the waveguide. The first and second beamsof image light may carry a color channel of the image.

In some embodiments, a second diffraction grating may be provided in thewaveguide. The first and second diffraction gratings may be disposed inthe waveguide between the first and second optical surfaces and offsetlaterally from each other. The first diffraction grating may include aplurality of volume Bragg gratings (VBGs) configured to expand the firstand second beams along the first axis and to redirect the first andsecond beams towards the second diffraction grating. The seconddiffraction grating may include a plurality of VBGs configured toreceive the first and second beams from the first diffraction grating,to expand the first and second beams along a second axis, and toout-couple the first and second beams from the waveguide for observationof the first and second portions of the FOV of the image by the user.

In some embodiments, projections of the first and second diffractiongratings onto the first optical surface are non-overlapping. The firstand second input ports may be coupled to the first optical surface onopposite sides of the waveguide, and each one of the first and seconddiffraction gratings may be symmetric with respect to an axisequidistant from the first and second input ports.

The first and second beams of image light may carry at least one colorchannel of the image. In embodiments where more than one color channelis present, the waveguide may further include third and fourth inputports for receiving third and fourth beams of image light carrying thefirst and second portions, respectively, of the FOV of the image, thethird and fourth beams of image light carrying a second color channel ofthe image. A third diffraction grating may be disposed in the waveguidebetween the first and second optical surfaces and offset laterally fromthe first and second diffraction gratings. The third diffraction gratingmay include a plurality of VBGs configured to expand the third andfourth beams along the first axis and to redirect the third and fourthbeams towards the second diffraction grating. The VBGs of the seconddiffraction grating may be configured to receive the third and fourthbeams from the third diffraction grating, to expand the third and fourthbeams along the second axis, and to out-couple the third and fourthbeams from the waveguide for observation of the image by the user.

In embodiments where at least three color channels are present, thewaveguide may further include fifth and sixth input ports for receivingfifth and sixth beams of image light carrying the first and secondportions, respectively, of the FOV of the image, the fifth and sixthbeams of image light carrying a third color channel of the image. Afourth diffraction gratings may be disposed in the waveguide between thefirst and second optical surfaces and offset laterally from the first tothird diffraction gratings. The fourth diffraction grating may include aplurality of VBGs configured to expand the fifth and sixth beams alongthe first axis and to redirect the fifth and sixth beams towards thesecond diffraction grating. The VBGs of the second diffraction gratingmay be configured to receive the fifth and sixth beams from the fourthdiffraction grating, to expand the fifth and sixth beams along thesecond axis, and to out-couple the fifth and sixth beams from thewaveguide for observation of the image by the user.

In accordance with the present disclosure, there is provided a waveguidefor conveying image light. The waveguide includes a first input port forreceiving a first beam of image light carrying an image in a firstwavelength band, opposed first and second outer optical surfaces forpropagating the first beam therebetween, and first and seconddiffraction gratings disposed in the waveguide between the first andsecond optical surfaces and offset laterally from each other. The firstdiffraction grating may include a plurality of VBGs configured to expandthe first beam along a first axis and to redirect the first beam towardsthe second diffraction grating. The second diffraction grating mayinclude a plurality of VBGs configured to receive the first beam fromthe first diffraction grating and to out-couple different portions ofthe first wavelength band of the first beam along a second axis, therebyexpanding the first beam along the second axis for observation of theimage by a user.

In some embodiments, projections of the first and second diffractiongratings onto the first optical surface are non-overlapping. The firstdiffraction grating may include e.g. between 300 and 1000 VBGs, and thesecond diffraction grating may include e.g. between 10 and 200 VBGs. Thefirst wavelength band may correspond to a color channel of the image.

In some embodiments, a second input port may be provided in thewaveguide for receiving a second beam of image light carrying the imagein a second wavelength band. A third diffraction grating may be disposedin the waveguide between the first and second optical surfaces andoffset laterally from the first and second diffraction gratings. Thethird diffraction grating may include a plurality of VBGs configured toexpand the second beam along the first axis and to redirect the secondbeam towards the second diffraction grating. The VBGs of the seconddiffraction grating may be configured to receive the second beam fromthe third diffraction grating and to out-couple different portions ofthe second wavelength band of the second beam along the second axis,thereby expanding the second beam along the second axis for observationof the image by the user.

A third input port may be further provided in the waveguide forreceiving a third beam of image light carrying the image in a thirdwavelength band. A fourth diffraction grating may be disposed in thewaveguide between the first and second optical surfaces and offsetlaterally from the first to third diffraction gratings. The fourthdiffraction grating may include a plurality of VBGs configured to expandthe third beam along the first axis and to redirect the third beamtowards the second diffraction grating. The VBGs of the seconddiffraction grating may be configured to receive the third beam from thefourth diffraction grating and to out-couple different portions of thethird wavelength band of the third beam along the second axis, therebyexpanding the third beam along the second axis for observation of theimage by the user. The first, second, and third input ports may beoffset from each other along the second axis, and the first, second, andthird wavelength bands may correspond to first, second, and third colorchannels of the image, respectively. The VBGs of the first and seconddiffraction gratings may be disposed in a same layer spaced apart fromthe first and second optical surfaces.

In at least some of the above embodiments, the VBGs of the firstdiffraction grating may have grating periods spatially varying along thefirst axis, e.g. within a range of 100 nm to 500 nm. The VBGs of thesecond diffraction grating may also have grating periods spatiallyvarying along the second axis, e.g. within a range of 100 nm to 300 nm.In embodiments where the VBGs of the first diffraction grating areconfigured to redirect the first and second beams of image light byreflective diffraction, the VBGs of the first diffraction grating mayinclude a plurality of fringes forming an angle with the first opticalsurface of between 34 degrees and 54 degrees, for example. Inembodiments wherein the VBGs of the first diffraction grating areconfigured to redirect the first and second beams of image light bytransmissive diffraction, the VBGs of the first diffraction grating mayinclude a plurality of fringes forming an angle with the first opticalsurface of greater than 80 degrees, for example. In any of the aboveembodiments, the VBGs of the second diffraction grating may include aplurality of fringes forming an angle with the first optical surface ofbetween 20 degrees and 38 degrees, or between 50 degrees and 70 degrees,for example. In embodiments where the first and second beams carry theimage in a first wavelength band corresponding to a color channel, theVBGs of the second diffraction grating may be configured to receive thefirst and second beams from the first diffraction grating and toout-couple different portions of the first wavelength band of the firstand second beams along the second axis, thereby expanding the first andsecond beams along the second axis.

Referring now to FIGS. 1A and 1B, a near-eye display (NED) 100 includesa waveguide 102 optically coupled to a projector 104. The waveguide 102is configured for conveying a beam 111 of image light emitted by theprojector 104. The waveguide 102 can be based on a transparent flat slabor plate having opposed first 131 and second 132 outer optical surfacesfor propagating the beam 111 between the optical surfaces 131, 132, e.g.in a zigzag pattern by total internal reflection (TIR). An input port121 may be provided for receiving the beam 111. An input coupler, suchas a prism 177, may be placed at the input port 121 for coupling thebeam 111 into the waveguide 102 for subsequent propagation in thewaveguide 102. The optical surfaces 131, 132 may include outsideparallel surfaces of the transparent plate, outside surfaces of volumeBragg gratings (VBGs), or surface-relief gratings (SRGs), for example.In some embodiments, the first 131 and second 132 optical surfaces maybelong to different parallel substrates separated by an air gap in whichthe beam 111 propagates.

A first diffraction grating 141 of waveguide 102 may include SRGs, VBGs,or both types of gratings. The first diffraction grating 141 isconfigured to expand the beam 111 along a first axis 151. Herein, theterm “to expand the beam along an axis” means that a projection of thebeam 111 onto the first axis 151 is expanded. It is to be noted that forthe projection to be expanded, the beam does not have to expand exactlyparallel to the axis 151, but the direction of beam 111 expansion mayform an angle, e.g. less than 45 degrees, to that axis 151, such thatthe projection of the beam 111 on the axis 151 is expanded. For example,in FIG. 1A, different beam portions 111A, 111B, 111C may expand alongdirections forming acute angle with the first axis 151. Note that thefirst axis 151 simply refers to an orientation, that is, horizontalorientation in FIG. 1A. The orientation may also be referenced relativeto an edge of the waveguide 102 or to another axis, for example a secondaxis 152, which is disposed vertically in FIG. 1A and perpendicular tothe first axis 151.

The expanded beam portions 111A, 111B, 111C are eventually directed bythe first diffraction grating 141 towards a second diffraction grating142, which is offset downwards in FIG. 1A, i.e. along the second axis152, relative to the first diffraction grating 141. The first 141 andsecond 142 diffraction gratings may be offset entirely, such that theirprojections on the first optical surface 131 do not overlap i.e. asshown in FIG. 1A, or they may be offset and overlap only partially. Thefirst 141 and second 142 diffraction gratings may be disposed in thewaveguide 102 between the first 131 and second 132 outer opticalsurfaces, e.g. offset to be adjacent respective first 131 and second 132optical surfaces as shown in FIG. 1B, centered in the waveguide 102, ordisposed at an arbitrary depth in the waveguide 102. The seconddiffraction grating 142 may include SRGs, VBGs, or both types ofgratings. The second diffraction grating 142 is configured to receivethe beam 111 from the first diffraction grating 141, to expand the beam111 along the second axis 152, and to out-couple the beam 111 from thewaveguide 102 for observation of the first and second portions of theFOV of the image by a user's eye 106 located at an eyebox 108.Throughout this disclosure, the term “eyebox” refers to a geometricalthree-dimensional (3D) area where the displayed image has an acceptablequality.

Referring to FIG. 2A, a waveguide 202 is an embodiment of the waveguide102 of FIGS. 1A and 1B. The waveguide 202 of FIG. 2A includes an inputport 221, a first diffraction grating 241, and a second diffractiongrating 242. The beam 111 is coupled into the waveguide 202 at the inputport 221. The first diffraction grating 241 includes a first pluralityof VBGs having fringes 261 shown by long-dash lines. The fringes 261 ofVBGs of the first diffraction grating 241 are configured to expand thebeam 111 along X-axis, and to direct the beam 111 towards the seconddiffraction grating 242, as shown schematically with rays 211. It isnoted that the term “expand the beam 111 along X-axis” includes caseswhere portions of the beam 111 propagate at an angle to the X-axis,similar to what was explained above with reference to FIG. 1A. The term“expanding a beam along an axis” throughput this disclosure generallyincludes beams expanded at an angle to the axis. For example, rays 211in FIG. 2A propagate in the first diffraction grating 241 at variousangles to the X-axis. It is further noted that the rays 211 of the beam111 propagate in a zigzag pattern between the first 131 and second 132optical surfaces by TIR from the first 131 and second 132 opticalsurfaces. The zigzag patterns are seen as straight lines in FIG. 2A,because they are viewed from top in FIG. 2A. In the embodiment shown inFIG. 2A, the period of the fringes 261, measured along a correspondingk-vector of the grating, varies from about 158 nm to 411 nm. The fringeperiod variation is required so that the rays 211 have a sharperdiffraction angle on the right end of the first diffraction grating 241(shorter periods) as compared to the left end of the first diffractiongrating 241 (longer periods). Grating period is chosen such that Braggconditions are satisfied for rays 211 at various locations and at allthe display wavelengths. Typically, the grating periods of the firstdiffraction grating 241 VBGs may vary within a range of 100 nm to 500nm.

The second diffraction grating 242 includes a second plurality of VBGshaving fringes 262 shown by short-dash lines. The VBGs of the seconddiffraction grating 242 are configured to receive the beam 111 from thefirst diffraction grating 241, to expand the beam 111 along Y-axis, andto out-couple the beam 111 from the waveguide 202 for observation of theimage carried by the beam 111 by a user. Eyebox 208 is typically smallerin size than the second diffraction grating 242. Solid line 207 on thefirst diffraction grating 241 denotes a boundary of the rays 211reaching the user's eye placed at center of eyebox with coordinate of(0,0, Z), where Z is eye relief distance, normally 15-20 mm. The periodof the fringes 262 of the second diffraction grating 242 varies fromabout 152 nm to 291 nm. The fringe period variation is required toexpand the output pupil along Y-axis by using wavelength division pupilexpansion, which will be described further below. Typically, the gratingperiods of the VBGs of the second diffraction grating 242 may varywithin a range of 100 nm to 300 nm.

The fringes 261 of the VBGs of the first diffraction grating 241 areoriented at approximately 24 degrees with respect to Y-axis and have atilt with respect to first 231 and second 232 optical surfaces of thewaveguide of about 44 degrees, as shown in FIG. 2B. The tilt angle maybe between 34 degrees and 54 degrees. In this tilt angle range, the VBGsredirect the beam 111 of image light primarily by reflectivediffraction.

The fringes 262 of the second plurality of VBGs are oriented atapproximately 104 degrees with respect to Y-axis and have a tilt withrespect to first 231 and second 232 optical surfaces of the waveguide ofabout 28 degrees, as shown in FIG. 2C. In a typical embodiment, theangle may be between 20 degrees and 38 degrees. At thickness of thewaveguide 202 of 1.5 mm, the first 241 and second 242 diffractiongratings may have a thickness of about 0.5 mm or more, that is, aboutone third of the waveguide 202 thickness, or more. The first 241 andsecond 242 diffraction gratings may be disposed in a same layer spacedapart from the first 231 and second 232 optical surfaces. The overallsize of the waveguide 202 of FIG. 2A, having reflective first 241 andsecond 242 diffraction gratings, is 70×70 mm.

Turning to FIG. 3, a diffraction efficiency spectrum of a typical VBG ofthe first 241 or second 242 diffraction gratings includes sharp peaks300 of high efficiency, the peaks having a spectral width of about 0.2nm. The peaks are separated by approximately 3.5 nm wide areas 302 oflow diffraction efficiency. At a given wavelength, the diffractionefficiency also depends on angle of incidence. To provide highefficiency in a wavelength range of a typical color channel, e.g. 20 nm,in a field of view (FOV) of several tens of degrees in X- andY-direction, many VBGs may need to be formed. By way of a non-limitingexample, the first diffraction grating 241 may include between 300 and1000 VBGs. For augmented reality (AR) applications, it may be desirableto limit or reduce the number of VBGs in the second diffraction grating242, because the second diffraction grating 242 is disposed againstuser's eye, and the user views the outside world through the seconddiffraction grating 242. Many VBGs in the view of the eye can make theviews of the outside objects hazy or color-fringed, and they can alsoreduce the contrast of the displayed virtual world image. For at leastthese reasons, it may be preferable to limit the number of VBGs in thesecond diffraction grating 242, e.g. to between 10 and 200 VBGs.

In accordance with the present disclosure, the number of VBGs in thesecond diffraction grating 242 required for good image quality in theeyebox 208 may be reduced by allowing light at different wavelengths tobe out-coupled from the waveguide 202 at different locations of theeyebox 208. Referring to FIG. 4A, an example of such a sparsediffraction grating 400 is shown in cross-section. The diffractiongrating 400 has a thickness t, extends along Y-axis, and has a VBGperiod varying along the Y-axis. Image light 411 destined for an eyebox408 is out-coupled at a location 402 of the diffraction grating 400 atan out-coupling angle θ which depends on wavelength and VBG period atthat location 402. Such a dependence is shown in FIG. 4B, where theout-coupling angle θ is plotted as a function of wavelength fordifferent VBG periods. To obtain a range of grating periods at thelocation 402, a range of out-coupling angles θ is first determined basedon the required size of the eyebox 408 and the eye relief, which isapproximately equal to the distance between the diffraction grating 400and the eyebox 408. In the example illustrated in FIG. 4A, theout-coupling angle θ ranges from θ₁=5 degrees to θ₂=23 degrees. Once thevalues of θ₁ and θ₂ at the location 402 are determined, FIG. 4B isconsulted to obtain a corresponding range of VBG periods. In FIG. 4B,different slanted lines 407 represent different VBG periods varying from310 nm to 620 nm along Y-axis. In this example, VBG periods at thelocation 402 need to cover a range from 360 nm to 590 nm for the imagelight 411 in the wavelength band of 450 nm to 630 nm to be diffractedout to the eyebox 408. VBGs with 360 nm grating period can diffract blueimage light at 460 nm to the eyebox 408 at θ=5 degrees, and VBGs with380 nm grating period can diffract beam wavelength of 460 nm to theeyebox 408 at θ=10 degrees; this same grating can diffract 480 nmwavelength light at θ=5 degrees, and so on. There is a small VBG periodchange from the location 402 to a neighboring location. The small VBGperiod change can cause small out-coupling wavelength shift at differentlocations of the eyebox 408 for a fixed FOV angle θ. While this mayresult in a slight color shift across the eyebox 408, this color shiftmay be acceptable when the wavelength band is narrow enough and belongsto a single color channel of the image to be displayed. By way of anon-limiting example, for a red color channel, a wavelength band ofbetween 620nm and 660 nm may be selected—light at any of thesewavelengths is generally perceived as red. The pupil expansion bywavelength division described herein has the advantage of reduced numberof VBGs required to cover the FOV of interest. For example, as few as 10to 200 VBGs per color channel may be required for a single colorchannel.

The principle of pupil expansion by wavelength division is furtherillustrated in FIG. 5. An NED 500 includes a waveguide 502 coupled to animage projector 504. The waveguide 502 includes first 541 and second 542diffraction gratings. The first diffraction grating 541 spreads an imagebeam 511 in a direction perpendicular to the plane of FIG. 5, and thesecond diffraction grating 542 spreads the image beam 511 vertically inFIG. 5. For red (R) channel, light 581 at a first red wavelength λ_(R1)is out-coupled at a first location 571; light 582 at a second redwavelength λ_(R2) is out-coupled at a second location 572; light 583 ata third red wavelength λ_(R3) is out-coupled at a third location 573;and light 584 at a fourth red wavelength λ_(R4) is out-coupled at afourth location 574. It is to be understood that the wavelengths λ₁, λ₂,λ_(R3), and λ₄ are center wavelengths of a rather broad wavelength band,i.e. at the first location 571, the light 581 occupies a wavelength bandof e.g. 600 nm to 640 nm; at the second location 572, the light 582occupies a wavelength band of e.g. 601 nm to 641 nm; at the thirdlocation 573, the light 583 occupies a wavelength band of e.g. 602 nm to642 nm, and so on, such that a color shift can be rather small whencompared to the wavelength bandwidth, which further reduces a perceivedcolor shift across the eyebox 408. The out-coupling of green (G) andblue (B) channels may be configured similarly, overlapping with theout-coupling of the R channel.

Referring to FIG. 6, a waveguide 602 is an embodiment of the waveguide102 of FIGS. 1A and 1B, and the waveguide 202 of FIG. 2A. The waveguide602 of FIG. 6 includes a first diffraction grating 641 and a seconddiffraction grating 642, each comprising a plurality of VBGs in asimilar manner as the first diffraction grating 241 and the seconddiffraction grating 242 of FIG. 2A; the fringes of the VBGs are notillustrated in FIG. 6 for simplicity. The VBGs of the first diffractiongrating 641 are configured to expand an image beam 611 along X-axis andto direct the image beam 611 towards the second diffraction grating 642,as shown schematically with individual rays 621 of the image beam 611.The VBGs of the second diffraction grating 642 are configured to receivethe beam 611 from the first diffraction grating 641, to expand the beam611 along Y-axis, and to out-couple the beam 611 from the waveguide 602for observation of the image carried by the beam 611 by a user. Solidline 607 on the first diffraction grating 641 denotes a boundary of therays 621 reaching the user's eye placed at the center of an eyebox 608.

The VBG fringes of the first diffraction grating 641 form an angle of 34degrees with Y-axis and are oriented at about 90 degrees w.r.t. theoptical surfaces of the waveguide 602. At angles of greater thanapproximately 80 degrees, the VBG fringes of the first diffractiongrating 641 redirect the first and second beams of image light mostly bytransmissive diffraction (transmissive grating configuration). The VBGfringes of the second diffraction grating 642 form an angle of 94degrees with Y-axis and are oriented at about 59 degrees w.r.t. theoptical surfaces of the waveguide 602. More generally, the VBG fringesof the second diffraction grating 642 may form an angle with the opticalsurfaces of the waveguide 602 of between 50 degrees and 70 degrees. Theoverall size of the waveguide 602 of FIG. 6 is 45×60 mm, which is only55% of the waveguide 202 of FIG. 2A by area.

The period of the fringes of the first 641 and second 642 diffractiongratings is spatially varying. The spatial variation of the gratingperiods is illustrated in FIGS. 7A and 7B, which show density maps ofthe fringes for both diffraction gratings 641 and 642. FIG. 7A shows amaximum fringe period map 741A for the first diffraction grating 641 anda maximum fringe period map 742A for the second diffraction grating 642,both measured along the grating vector Kg. FIG. 7B shows a minimumfringe period map 741B for the first diffraction grating 641 and amaximum fringe period map 742B for the second diffraction grating 642,both measured along the grating vector Kg.

Turning to FIG. 8, a waveguide 802 is similar to the waveguide 292 ofFIG. 2A with reflective VBGs in the first diffraction grating 241. Thewaveguide 802 of FIG. 8 includes first 821, second 822, and third 823input ports for receiving first 811, second 812, and third 813 beams ofimage light carrying the image in first, second, and third wavelengthbands respectively, and a pair of outer optical surfaces for propagatingimage light between the surfaces. The first 821, second 822, and third823 input ports may be offset in a direction of Y-axis as shown. A firstdiffraction grating 841 includes a plurality of VBGs configured toexpand the first beam 811 along the X-axis and to redirect the firstbeam 811 towards a second diffraction grating 842. A third diffractiongrating 843 includes a plurality of VBGs configured to expand the secondbeam 812 along the-X-axis and to redirect the second beam 812 towardsthe second diffraction grating 842. A fourth diffraction grating 844includes a plurality of VBGs configured to expand the third beam 813along the X-axis and to redirect the third beam 813 towards the seconddiffraction grating 842. The first, second, and third wavelength bandsmay correspond to red (R), green (G), and blue (B) channels of theimage.

The second diffraction grating 842 includes a plurality of VBGsconfigured to receive the first 811, second 812, and third 813 beamsfrom respectively the first 841, third 843, and fourth 844 diffractiongratings and to out-couple different portions of the respective first,second, and third wavelength bands along the Y-axis, thereby expandingthe first 811, second 812, and third 813 beams along the Y-axis forobservation of the image by a user at an eyebox 808, as has beenexplained above w.r.t. FIG. 5. At least two top gratings, e.g. the firstdiffraction grating 841 (FIG. 8) and the third diffraction grating 843coupled to the first 821 and second 822 input ports respectively, may beprovided. To have different redirection angles for the left and rightsides of the first 841, second 842, and fourth 844 diffraction gratings,the VBGs of these gratings may have grating periods spatially varying ingoing from left to right, i.e. along the X-axis.

The VBGs of the first 841, third 843, and fourth 844 diffractiongratings include grooves at an angle of approximately 21 degrees w.r.t.the Y-axis, tilted at about 47 degrees w.r.t. the waveguide 802 surface.The VBGs of the second diffraction grating 842 include grooves at anangle of approximately 111 degrees w.r.t. the Y-axis, tilted at about 29degrees w.r.t. the waveguide 802 surface. At these tilt angles, the VBGsredirect the beams 811, 812, and 813 of image light primarily byreflective diffraction; a diagonal FOV of 60 degrees is provided at thesize of a grating area of the waveguide 802 of only 75×62 mm.

Referring to FIG. 9, a waveguide 902 includes “transmissive diffraction”top gratings, and is otherwise similar to the waveguide 802 of FIG. 8.The waveguide 902 of FIG. 9 includes first 921, second 922, and third923 input ports for receiving first 911, second 912, and third 913 beamsof image light carrying the image in first, second, and third wavelengthbands respectively, and a pair of outer optical surfaces for propagatingimage light between the surfaces. The first 921, second 922, and third923 input ports may be offset in a direction of Y-axis, as shown. Afirst diffraction grating 941 includes a plurality of VBGs configured toexpand the first beam 911 generally along X-axis and to redirect thefirst beam 911 towards a second diffraction grating 942. A thirddiffraction grating 943 includes a plurality of VBGs configured toexpand the second beam 912 generally along the X-axis and to redirectthe second beam 912 towards the second diffraction grating 942. A fourthdiffraction grating 944 includes a plurality of VBGs configured toexpand the third beam 913 generally along the X-axis and to redirect thethird beam 913 towards the second diffraction grating 942.

The second diffraction grating 942 includes a plurality of VBGsconfigured to receive the first 911, second 912, and third 913 beamsfrom respectively the first 941, third 943, and fourth 944 diffractiongratings and to out-couple different portions of the respective first,second, and third wavelength bands along the Y-axis, thereby expandingthe first 911, second 912, and third 913 beams along the Y-axis forobservation of the image by a user at an eyebox 908.

The VBGs of the first 941, third 943, and fourth 944 diffractiongratings include grooves at an angle of approximately 30 degrees w.r.t.the Y-axis, tilted at about 90 degrees w.r.t. the waveguide 802 surface.At these tilt angles, the VBGs redirect the beams 911, 912, and 913 ofimage light primarily by transmissive diffraction. The VBGs of thesecond diffraction grating 842 include grooves at an angle ofapproximately 102 degrees w.r.t. the Y-axis, tilted at about 60 degreesw.r.t. the waveguide 802 surface; a diagonal FOV of 60 degrees isprovided at the size of the waveguide 802 of 60×70 mm.

Turning to FIG. 10, a required horizontal 1001 and vertical 1002waveguide size is plotted against a diagonal FOV in degrees at 12×10 mmeyebox and 4:3 aspect ratio. The refractive index of the waveguide istaken to be 1.5. It is seen that the required diagonal FOV is a primaryfactor driving overall waveguide size at a given size of the eyebox.

In accordance with an aspect of this disclosure, one may reduce overallwaveguide size by segmenting the image FOV and providing differentoptical input ports for inputting optical signals carrying the differentFOV segments. By way of a non-limiting, illustrative example, a NED 1100of FIGS. 11A and 11B includes not one but two projectors, a firstprojector 1104 and a second projector 1105 (FIG. 12B). The first 1111and second 1112 beams of image light emitted by the first 1104 andsecond 1105 projectors carry first and second portions, respectively, ofthe FOV of the image, e.g. conterminous or partially overlappingportions of the FOV. A waveguide 1102 is optically coupled to the first1104 and a second 1105 projectors at first 1121 and second 1122 inputports, respectively. The first 1121 and second 1122 input ports aredisposed at the first optical surface 1131 at opposite sides of thewaveguide 1102, i.e. at left and right sides in FIGS. 11A and 11B. Thewaveguide 1102 can be based on a transparent flat plate or slab havingopposed first 1131 and second 1132 outer optical surfaces forpropagating the first 1111 and second 1112 beams therebetween. A firstdiffraction grating 1141 includes a first portion 1191 (solid outline)configured to expand the first beam 1111 along a first axis 1151, and asecond portion 1192 (dashed outline) configured to expand the secondbeam 1112 along the first axis 1151. The first 1191 and second 1192portions may overlap as shown. The first 1111 and second 1112 beams arethen out-coupled from the waveguide 1102 for observation of the firstand second portions of the FOV of the image at an eyebox 1108 by auser's eye 1106. The beams 1111, 1112 may be out-coupled by a seconddiffraction grating 1142.

In the embodiment shown in FIGS. 11A and 11B, the first 1141 and second1142 diffraction gratings are disposed in the waveguide 1102 between thefirst 1131 and second 1132 optical surfaces of the waveguide 1102 andare offset laterally from each other as shown. The first 1141 and second1142 diffraction gratings are not overlapping, i.e. their projectionsonto the first 1131 or second 1132 surfaces are not overlapping oneanother, although in other embodiments, they may overlap. The firstdiffraction grating 1141 of FIG. 11A includes a plurality of VBGsconfigured to expand the first 1111 and second 1112 beams along thefirst axis 1151 and to redirect the first 1111 and second 1112 beamstowards the second diffraction grating 1142. The second diffractiongrating 1142 includes a plurality of VBGs configured to receive thefirst 1111 and second 1112 beams from the first diffraction grating1141, to expand the first 1111 and second 1112 beams along a second axis1152, and to out-couple the first 1111 and second 1112 beams from thewaveguide 1102 for observation of the first and second portions of theFOV of the image by a user's eye 1106 at an eyebox 1108. At least one,or both diffraction gratings 1141 and 1142 may be symmetric with respectto an axis 1153 equidistant from the first 1121 and second 1122 inputports, although a strict symmetry is not required.

Referring to FIG. 12, a waveguide 1202 is an embodiment of the waveguide1102 of FIGS. 11A and 11B. The waveguide 1202 of FIG. 12 includes afirst input port 1221, a second input port 1222, a first diffractiongrating 1241, and a second diffraction grating 1242. The first beam 1111is coupled into the waveguide 1202 at the first input port 1221, and thesecond bean 1112 is coupled into the waveguide 1202 at the second inputport 1222. The first diffraction grating 1241 includes a first portion1291 having a plurality of VBGs with fringes 1261 shown by dashed lines,and a second portion 1292 having a plurality of VBGs with fringes 1262shown by solid lines. The fringes 1261 of the first portion areconfigured to expand the first beam 1111 along X-axis and to direct thefirst beam 1111 towards the second diffraction grating 1242. Similarly,the fringes 1262 of the second portion 1292 are configured to expand thesecond beam 1112 along the X-axis and to direct the second beam 1112towards the second diffraction grating 1242.

In the embodiment shown in FIG. 12, the first input port 1221 isdisposed at a left side of the waveguide 1202 for in-coupling imagelight directed to a right half of the FOV, and the second input port1222 is disposed at a right side of the waveguide 1202 for in-couplingimage light directed to a left half of the FOV. The period of thefringes of the VBGs of the first 1291 and second 1292 portions of thefirst diffraction grating 1241, as measured along a correspondingk-vector of the first diffraction grating 1241, varies from 163 nm to337 nm. The fringe period variation is required so that the beams 1111,1112 have a sharper diffraction angle on ends opposite to respectiveinput ports 1221, 1222 (shorter periods) as compared to the ends closeto the input ports 1221, 1222 (longer periods). The fringes 1261, 1262of the VBGs are oriented at approximately 50 degrees with respect toY-axis and have a tilt with respect to optical surfaces of the waveguide1202 of about 48 degrees. At these tilt angles, the VBGs redirect thefirst 1111 and second 1112 beams of image light primarily by reflectivediffraction.

The second diffraction grating 1242 may also have two portions, 1281 and1282, having fringes 1271 (dashed lines) and 1272 (solid lines),respectively, for expanding the first 1111 and second 1112 beams,respectively, along Y-axis and for outputting the first 1111 and second1112 beams at an eyebox 1208 for observation by a user. The fringeperiod of the fringes 1271 and 1272 of the second diffraction gratingvaries from 153 nm to 294 nm to provide pupil expansion by wavelengthdivision, as described above with reference to FIGS. 4A, 4B, and FIG. 5.The fringes 1271, 1272 (FIG. 12) are oriented at approximately 74degrees with respect to Y-axis and have a tilt with respect to opticalsurfaces of the waveguide 1202 of about 30 degrees. The overall size ofa grating area of the waveguide 1202 is 48×60 mm.

In some embodiments, the input ports 1221 and 1222 are in-coupling twoportions of a FOV of a same single color channel, and different inputports are provided for different color channels, similarly to thewaveguide 802 of FIG. 8. FIG. 13A illustrates such an embodiment. Awaveguide 1302A has first 1321 and second 1322 input ports for receivingfirst 1311 and second 1312 beams of image light carrying the first andsecond portions, respectively, of the FOV of a blue (B) channel of theimage; third 1323 and fourth 1324 input ports for receiving third 1313and fourth 1314 beams of image light carrying the first and secondportions, respectively, of the FOV of a green (G) channel; and fifth1325 and sixth 1326 input ports for receiving fifth 1315 and sixth 1316beams of image light carrying the first and second portions,respectively, of the FOV of a red (R) channel of the image to bedisplayed. A first diffraction grating 1341 is disposed between theoptical surfaces of the waveguide 1302A and includes portions 1391 and1392 with VBGs configured to expand the first 1311 and second 1312beams, respectively, along X-axis and to direct the first 1311 andsecond 1312 beams towards a second diffraction grating 1342A. A thirddiffraction grating 1343 is disposed between the optical surfaces of thewaveguide 1302A and is offset laterally from the first 1341 and second1342A diffraction gratings. The third diffraction grating includesportions 1393 and 1394 with VBGs configured to expand the third 1313 andfourth 1314 beams, respectively, along X-axis and to redirect the third1313 and fourth 1314 beams towards the second diffraction grating 1342A.Similarly, a fourth diffraction grating 1344 may be disposed between theoptical surfaces of the waveguide 1302A and is offset laterally from thefirst 1341, second 1342A, and third 1343 diffraction gratings. Thefourth diffraction grating 1344 includes portions 1395 and 1396 withVBGs configured to expand the fifth 1315 and sixth 1316 beams,respectively, along X-axis and to redirect the fifth 1315 and sixth 1316beams towards the second diffraction grating 1342A.

The VBGs of the second diffraction grating 1342A are configured toreceive the first 1311 and second 1312 beams from the first diffractiongrating 1341, the third 1313 and fourth 1314 beams from the thirddiffraction grating 1343, and the fifth 1315 and sixth 1316 beams fromthe fourth diffraction grating 1344; to expand the beams 1311-1316 alongthe Y-axis; and to out-couple the beams 1311-1316 from the waveguide1302A at an eyebox 1308 for observation of the image by the user. Thelight from left-side first 1321, third 1323, and fifth 1325 input portsis sent to a right half of the FOV at the eyebox 1308, and the lightfrom right-side second 1322, fourth 1324, and sixth 1326 input ports issent to a left half of the FOV at the eyebox 1308 in FIG. 13A.

Fringes of the VBGs of the first 1341, third, 1343, and fourth 1344diffraction gratings are oriented at approximately 45 degrees withrespect to Y-axis and have a tilt with respect to the optical surfacesof the waveguide 1302A of about 51 degrees. At these tilt angles, theVBGs redirect the beams 1311-1316 of image light primarily by reflectivediffraction. The VBG fringes of the second diffraction grating 1342A areoriented at approximately 75 degrees with respect to Y-axis and have atilt with respect to the optical surfaces of the waveguide 1302A ofabout 60 degrees. The overall size of a grating area of the waveguide1302A is ˜50×50 mm. The waveguide 1302A size can be reduced due to acompact placement of the waveguides.

Turning to FIG. 13B, a waveguide 1302B is similar to the waveguide 1302Aof FIG. 13A, but has a different orientation of the VBG grooves, andslightly different shape. The top diffraction gratings 1340Bcorresponding to the first 1341, third 1343, and fourth 1344 diffractiongratings of FIG. 13A, have VBG fringes at 41 degrees w.r.t. Y-axis,tilted at 90 degrees w.r.t. the optical surfaces of the waveguide 1302B;this corresponds to a transmissive diffraction grating configuration. Abottom diffraction grating 1342B, corresponding to the seconddiffraction grating 1342A in FIG. 13A, has VBG fringes at 77 degreesw.r.t. Y-axis, tilted at 60 degrees w.r.t. the optical surfaces of thewaveguide 1302B. The image light is outputted at the eyebox 1308. Theoverall size of a grating area of the waveguide 1302B of FIG. 13B is˜50×45 mm. The waveguide 1302B size can be reduced due to more compactplacement of the waveguides.

Turning now to FIG. 13C, a waveguide 1302C is similar to the waveguide1302A of FIG. 13A, but has a different orientation of the VBG groovesand a slightly different shape. The top diffraction gratings 1340Ccorresponding to the first 1341, third 1343, and fourth 1344 diffractiongratings of FIG. 13A, have VBG fringes at 42 degrees w.r.t. Y-axis,tilted at 90 degrees w.r.t. the optical surfaces of the waveguide 1302B;this corresponds to a transmissive diffraction grating configuration. Abottom diffraction grating 1342C, corresponding to the seconddiffraction grating 1342A in FIG. 13A, has VBG fringes at 78 degreesw.r.t. Y-axis, tilted at 32 degrees w.r.t. the optical surfaces of thewaveguide 1302B. The image light is outputted at the eyebox 1308. Theoverall size of a grating area of the waveguide 1302C of FIG. 13C is˜50×45 mm. The waveguide 1302C size can be reduced due to a compactplacement of the waveguides.

Referring to FIG. 13D, a waveguide 1302D is similar to the waveguide1302A of FIG. 13A, but has a different orientation of VBG fringes, suchthat the light from left-side input ports is sent to a left half of theFOV at the eyebox 1308, and the light from right-side input ports issent to a right half of the FOV. The VBG fringes of top diffractiongratings 1340D are tilted w.r.t Y-axis of about 35 degrees and form anangle with top or bottom planes of the waveguide 1302D of about 51degrees (transmissive grating configuration); and the VBG fringes ofbottom diffraction gratings 1342D are tilted w.r.t Y-axis of about 106degrees and form an angle with top or bottom planes of the waveguide1302D of about 60 degrees (transmissive grating configuration). Thediagonal full FOV is about 60 degrees. The waveguide size depends on therequired diagonal FOV; the required horizontal and vertical waveguidesize may vary from 30-35 mm to about 65 mm to obtain the diagonal FOV inthe range of 35 to 75 degrees.

Referring to FIG. 13E, a waveguide 1302E is similar to the waveguide1302D of FIG. 13D. The VBG fringes of top diffraction gratings 1340E aretilted w.r.t Y-axis of about 47 degrees and form an angle with top orbottom planes of the waveguide 1302E of about 54 degrees (transmissivegrating configuration); and the VBG fringes of bottom diffractiongratings 1342E are tilted w.r.t Y-axis of about 100 degrees and form anangle with top or bottom planes of the waveguide 1302E of about 63degrees (transmissive grating configuration). The diagonal full FOV isabout 70 degrees. The waveguide size depends on the required diagonalFOV; the required horizontal and vertical waveguide size may vary from30-35 mm to about 55 mm to obtain the diagonal FOV in the range of 35 to70 degrees.

Referring to FIG. 13F, a waveguide 1302F is similar to the waveguide1302D of FIG. 13D. The VBG fringes of top diffraction gratings 1340F aretilted w.r.t Y-axis of about 36 degrees and form an angle with top orbottom planes of the waveguide 1302F of about 90 degrees (transmissivegrating configuration); and the VBG fringes of bottom diffractiongratings 1342F are tilted w.r.t Y-axis of about 105 degrees and form anangle with top or bottom planes of the waveguide 1302F of about 32degrees (reflective diffraction grating configuration). The diagonalfull FOV is about 70 degrees. The waveguide size depends on the requireddiagonal FOV; the required horizontal and vertical waveguide size mayvary from 30-35 mm to 60-68 mm to obtain the diagonal FOV in the rangeof 35 to 90 degrees.

Referring to FIG. 13G, a waveguide 1302G is similar to the waveguide1302F of FIG. 13F. The VBG fringes of top diffraction gratings 1340G aretilted w.r.t Y-axis of about 38 degrees and form an angle with top orbottom planes of the waveguide 1302G of about 90 degrees (transmissivegrating configuration); and the VBG fringes of bottom diffractiongratings 1342G are tilted w.r.t Y-axis of about 102 degrees and form anangle with top or bottom planes of the waveguide 1302G of about 35degrees (reflective grating configuration). The diagonal full FOV isabout 70 degrees. The waveguide size depends on the required diagonalFOV; the required horizontal and vertical waveguide size may vary from30-35 mm to 60-68 mm to obtain the diagonal FOV in the range of 35 to 90degrees. In FIGS. 13A to 13G, the FOV aspect ratio is 16:9.

FIGS. 14A, 14B, and 14C show possible pathways for external light toreflect from VBG fringes of the out-coupling diffraction gratingsthereby causing artifacts due to so-called “rainbow” effect. Themagnitude of the rainbow artifacts depends on density and orientation ofVBG fringes. Fringes 1499A, 1499B, and 1499C of the VBGs in a waveguide1400 can create rainbow paths 1401, 1402 (FIG. 14A); 1403, 1404 (FIG.14B); and 1405, 1406 (FIG. 14C) for light to reach the user's eye 1408.In the configurations presented, the waveguide 1302A of FIG. 13A doesnot exhibit any rainbow effects; the waveguide 1302B of FIG. 13B canhave rainbow path 1401 in the top diffraction gratings 1340B; andwaveguide 1302C of FIG. 13C (smallest of all three) can have rainbowpaths 1403 and 1404 in the bottom diffraction grating 1342C. Similarly,the waveguides 1302D of FIG. 13D and 1302E of FIG. 13E do not exhibitany rainbow effects; while the waveguides 1302 F of FIG. 13F and 1302Gof FIG. 13G can have rainbow paths 1403 and 1404 in the bottomdiffraction gratings 1342F and 1342G respectively, while providing alarger diagonal FOV. Thus, a tradeoff may exist between the overall sizeof the waveguide at a required FOV and the presence of rainbow effects;it is to be remembered that only rainbow paths capable of reaching theuser's eye 1408 need to be considered in practice.

Referring to FIGS. 15A and 15B, a near-eye artificial reality/virtualreality (AR/VR) display 1500 may include waveguides of the presentdisclosure, e.g. the waveguide 102 of FIGS. 1A and 1B, the waveguide 202of FIG. 2A, the waveguide 502 of FIG. 5, the waveguide 602 of FIG. 6,the waveguide 802 of FIG. 8, the waveguide 902 of FIG. 9, the waveguide1102 of FIG. 11, the waveguide 1202 of FIG. 12, and/or the waveguides1302A, 1302B, or 1302C of FIGS. 13A, 13B, and 13C respectively, to guideimage light to eyeboxes 1510 of the near-eye AR/VR display 1500. A bodyor frame 1502 of the near-eye AR/VR display 1500 may have a form factorof eyeglasses, as shown in this example. A display unit 1504 includes adisplay assembly 1506 (FIG. 15B) which provides image light 1508 to theeyebox 1510, i.e. a geometrical area where a good-quality image may bepresented to a user's eye 1512. The display assembly 1506 may include aseparate AR/VR display module for each eye, or one AR/VR display modulefor both eyes. For the latter case, an optical switching device may becoupled to a single electronic display for directing images to the leftand right eyes of the user in a time-sequential manner, one frame forleft eye and one frame for right eye. The images may be presented fastenough, i.e. with a fast enough frame rate, that the individual eyes donot notice the flicker and perceive smooth, steady images of surroundingvirtual or augmented scenery.

An electronic display of the display assembly 1506 may include, forexample and without limitation, a liquid crystal display (LCD), anorganic light emitting display (OLED), an inorganic light emittingdisplay (ILED), an active-matrix organic light-emitting diode (AMOLED)display, a transparent organic light emitting diode (TOLED) display, aprojector, or a combination thereof. The near-eye AR/VR display 1500 mayalso include an eye-tracking system 1514 for determining, in real time,a gaze direction and/or the vergence angle of the user's eyes 1512. Thedetermined gaze direction and vergence angle may also be used forreal-time compensation of visual artifacts dependent on the angle ofview and eye position. Furthermore, the determined vergence and gazeangles may be used for interaction with the user, highlighting objects,bringing objects to the foreground, dynamically creating additionalobjects or pointers, etc. Furthermore, the near-eye AR/VR display 1500may include an audio system, such as small speakers or headphones.

Turning now to FIG. 16, an HMD 1600 is an example of an AR/VR wearabledisplay system which encloses user's face, for a greater degree ofimmersion into the AR/VR environment. The HIVID 1600 may include any ofthe waveguides of the present disclosure to guide image light toeyeboxes. The HMD 1600 can present content to the user as a part of anAR/VR system, which may further include a user position and orientationtracking system, an external camera, a gesture recognition system,control means for providing user input and controls to the system, and acentral console for storing software programs and other data forinteracting with the user for interacting with the AR/VR environment.The function of the HIVID 1600 is to augment views of a physical,real-world environment with computer-generated imagery, and/or togenerate entirely virtual 3D imagery. The HIVID 1600 may include a frontbody 1602 and a band 1604. The front body 1602 is configured forplacement in front of eyes of a user in a reliable and comfortablemanner, and the band 1604 may be stretched to secure the front body 1602on the user's head. A display system 1680, including waveguidesdisclosed herein, may be disposed in the front body 1602 for presentingAR/VR imagery to the user. Sides 1606 of the front body 1602 may beopaque or transparent.

In some embodiments, the front body 1602 includes locators 1608, aninertial measurement unit (IMU) 1610 for tracking acceleration of theHIVID 1600, and position sensors 1612 for tracking position of the HIVID1600. The locators 1608 are traced by an external imaging device of anAR/VR system, such that the AR/VR system can track the location andorientation of the entire HIVID 1600. Information generated by the IMUand the position sensors 1612 may be compared with the position andorientation obtained by tracking the locators 1608, for improvedtracking of position and orientation of the HIVID 1600. Accurateposition and orientation is important for presenting appropriate virtualscenery to the user as the latter moves and turns in 3D space.

The HIVID 1600 may further include an eye tracking system 1614, whichdetermines orientation and position of user's eyes in real time. Theobtained position and orientation of the eyes allows the HIVID 1600 todetermine the gaze direction of the user and to adjust the imagegenerated by the display system 1680 accordingly. In one embodiment, thevergence, that is, the convergence angle of the user's eyes gaze, isdetermined. The determined gaze direction and vergence angle may also beused for real-time compensation of visual artifacts dependent on theangle of view and eye position. Furthermore, the determined vergence andgaze angles may be used for interaction with the user, highlightingobjects, bringing objects to the foreground, creating additional objectsor pointers, etc. An audio system may also be provided including e.g. aset of small speakers built into the front body 1602.

1. A waveguide for conveying image light, the waveguide comprising: afirst input port for receiving a first beam of image light carrying animage in a first wavelength band; opposed first and second outer opticalsurfaces for propagating the first beam therebetween; and first andsecond diffraction gratings disposed in the waveguide between the firstand second optical surfaces and offset laterally from each other;wherein the first diffraction grating comprises a plurality of volumeBragg gratings (VBGs) configured to expand the first beam along a firstaxis and to redirect the first beam towards the second diffractiongrating; and wherein the second diffraction grating comprises aplurality of VBGs configured to receive the first beam from the firstdiffraction grating and to out-couple different portions of the firstwavelength band of the first beam at different locations along a secondaxis, thereby expanding the first beam along the second axis forobservation of the image by a user.
 2. The waveguide of claim 1, whereinprojections of the first and second diffraction gratings onto the firstoptical surface are non-overlapping.
 3. The waveguide of claim 1,wherein the first diffraction grating comprises between 300 and 1000VBGs, and wherein the second diffraction grating comprises between 10and 200 VBGs.
 4. The waveguide of claim 1, wherein the first wavelengthband corresponds to a color channel of the image.
 5. The waveguide ofclaim 1, further comprising: a second input port for receiving a secondbeam of image light carrying the image in a second wavelength band; athird diffraction grating disposed in the waveguide between the firstand second optical surfaces and offset laterally from the first andsecond diffraction gratings; wherein the third diffraction gratingcomprises a plurality of VBGs configured to expand the second beam alongthe first axis and to redirect the second beam towards the seconddiffraction grating; and wherein the VBGs of the second diffractiongrating are configured to receive the second beam from the thirddiffraction grating and to out-couple different portions of the secondwavelength band of the second beam at different locations along thesecond axis, thereby expanding the second beam along the second axis forobservation of the image by the user.
 6. The waveguide of claim 5,further comprising: a third input port for receiving a third beam ofimage light carrying the image in a third wavelength band; a fourthdiffraction grating disposed in the waveguide between the first andsecond optical surfaces and offset laterally from the first to thirddiffraction gratings; wherein the fourth diffraction grating comprises aplurality of VBGs configured to expand the third beam along the firstaxis and to redirect the third beam towards the second diffractiongrating; and wherein the VBGs of the second diffraction grating areconfigured to receive the third beam from the fourth diffraction gratingand to out-couple different portions of the third wavelength band of thethird beam at different locations along the second axis, therebyexpanding the third beam along the second axis for observation of theimage by the user.
 7. The waveguide of claim 6, wherein the first,second, and third input ports are offset from each other along thesecond axis.
 8. The waveguide of claim 6, wherein the first, second, andthird wavelength bands correspond to first, second, and third colorchannels of the image, respectively.
 9. The waveguide of claim 1,wherein the VBGs of the first and second diffraction gratings aredisposed in a same layer spaced apart from the first and second opticalsurfaces.
 10. The waveguide of claim 1, wherein the VBGs of the firstdiffraction grating have grating periods spatially varying along thefirst axis.
 11. The waveguide of claim 10, wherein the grating periodsof the VBGs of the first diffraction grating are varying within a rangeof 100 nm to 500 nm.
 12. The waveguide of claim 1, wherein the VBGs ofthe second diffraction grating have grating periods spatially varyingalong the second axis.
 13. The waveguide of claim 12, wherein thegrating periods of the VBGs of the second diffraction grating arevarying within a range of 100 nm to 300 nm.
 14. The waveguide of claim1, wherein the VBGs of the first diffraction grating are configured toredirect the first and second beams of image light by reflectivediffraction.
 15. The waveguide of claim 14, wherein the VBGs of thefirst diffraction grating comprise a plurality of fringes forming anangle with the first optical surface of between 34 degrees and 54degrees.
 16. The waveguide of claim 15, wherein the VBGs of the seconddiffraction grating comprise a plurality of fringes forming an anglewith the first optical surface of between 20 degrees and 38 degrees. 17.The waveguide of claim 1, wherein the VBGs of the first diffractiongrating are configured to redirect the first and second beams of imagelight by transmissive diffraction.
 18. The waveguide of claim 17,wherein the VBGs of the first diffraction grating comprise a pluralityof fringes forming an angle with the first optical surface of greaterthan 80 degrees.
 19. The waveguide of claim 18, wherein the VBGs of thesecond diffraction grating comprise a plurality of fringes forming anangle with the second optical surface of between 50 degrees and 70degrees.
 20. The waveguide of claim 1, wherein the image has a field ofview (FOV), wherein the first beam of image light carries a firstportion of the FOV of the image, the waveguide further comprising asecond input port for receiving a second beam of image light carrying asecond portion of the FOV of the image in the first wavelength band;wherein the VBGs of the first diffraction grating are configured toexpand the second beam along the first axis and to redirect the secondbeam towards the second diffraction grating; and wherein the VBGs of thesecond diffraction grating are configured to receive the second beamfrom the first diffraction grating and to out-couple different portionsof the first wavelength band of the second beam at different locationsalong the second axis, thereby expanding the second beam along thesecond axis for observation of the image by the user.